Tails may have helped ancient animals move from water to land
Clambering out of the water and up sandy shorelines was a monumental feat for some of the first creatures to walk on land. These ancient animals needed all the help they could get to push beyond the water’s edge – and scientists say some of that help may have come from their tails.
After studying the movements of a unique fish called the African mudskipper, researchers realised that tails were an essential tool that could have allowed the first land dwellers to gain traction on the sloped, sandy shorelines they would have encountered upon leaving their watery homes.
The team reported its findings in yesterday’s edition of the journal Science.
‘‘There are a number of organisms that exist today that . . . could resemble those animals of the Devonian era,’’ which ended about 360 million years ago, said Daniel Goldman, a physicist at Georgia Tech who worked on the study.
The African mudskipper is a small amphibious fish that spends most of its life moving in and out of the water, taking care never to dry out. Roughly 10cm long, with two front legs and a tail, they can breathe through gills and through their skin, and they usually live in mudflats or mangrove estuaries.
As mudskippers climb out of the water, both front legs move in synchrony – reaching up and out to the side, then digging down into the sand to thrust their bodies forward. ‘‘They’re a fish that can do a push-up,’’ said study lead author Benjamin McInroe, a graduate student in biophysics at the University of California, Berkeley.
McInroe’s colleagues watched mudskippers as they moved across sand, allowing them to make consistent, repeatable measurements.
On level ground, the fish didn’t use their tails and mostly kept them straight back. But as the researchers increased the slope of the sandy beach, the mudskippers relied on their tails to propel themselves forward and to keep from sliding downhill. By doing this, they nearly doubled the distance they were able to cover with each stride.
Next, the researchers 3-D-printed a robot that used the same motion to navigate its way up tricky slopes. Instead of letting it loose in sand, which could damage its delicate motors, they tested it in an environment filled with poppy seeds or tiny plastic beads.
Watching the robot helped the researchers recognise some of the basic principles that would have helped ancient animals climb out of the water. They found that when the robot, or the mudskipper, couldn’t use its tail, there was little room for error. ‘‘On sandy ground, the tail was critical,’’ Goldman said. ‘‘It buffered against poor foot use and placement.’’
The researchers think that their results could shed light on the behaviour of early land dwellers.
Although the study provided some evidence that a tail would be helpful on difficult terrain, researchers should consider the body shapes of actual creatures from the fossil record, said Stephanie Pierce, a vertebrate paleontologist at Harvard who was not involved in the study. Only then could they come to any conclusions about how tails facilitated the transition from life in the water to life on land, she said.
The first vertebrates to emerge from the water had four legs, not two, Pierce said, and shorter tails relative to their overall body length.